A communication system is comprised, at a minimum, of a transmitter and a receiver interconnected by a communication channel. Communication signals formed at, or applied to, the transmitter are converted at the transmitter into a form to permit their transmission upon the communication channel. The receiver is tuned to the communication channel to receive the communication signals transmitted thereupon. Once received, the receiver converts, or otherwise recreates, the communication signal transmitted by the transmitter.
A radio communication system is a type of communication system in which the communication channel comprises a radio frequency channel formed of a portion of the electromagnetic frequency spectrum. A radio communication system is advantageous in that the transmitter and receiver need not be interconnected by way of wireline connections. As, instead, the communication channel is formed of a radio frequency channel, communication signals can be transmitted between the transmitter and the receiver even when wireline connections therebetween would be inconvenient or impractical.
The quality of communications in a communication system is dependant, in part, upon levels of noise superimposed upon the information signal transmitted by the transmitter to the receiver. Noise can be introduced upon the informational signal while being transmitted upon the communication channel, and once received at the receiver, at the transmitter. Noise can also be applied to the transmitter together with application to the transmitter of the information signal.
When the noise level of the signal provided to a listener positioned at the receiver is high relative to the informational signal, the audio quality of the signal provided to the listener is low. If the noise levels are too significant, the listener is unable to adequately understand the informational signal provided at the receiver. Noise can be either periodic or aperiodic in nature. Random noise and white noise are exemplary of aperiodic noise. While a human listener is generally able to fairly successfully "block out" aperiodic noise from an informational signal, periodic noise is sometimes more distracting to the listener.
Various manners by which to remove noise components superimposed upon an informational signal, or at least to improve the ratio of the level of the informational signal to the level of the noise, are sometimes utilized. For instance, filter circuits are sometimes used which filter or otherwise remove the noise components from a communication signal, both prior to transmission by a transmitter and also subsequent to reception at a receiver.
Conventional filter circuits include circuitry for filtering noise components superimposed upon an informational signal. A spectral subtraction process is performed during operation of some of such conventional filter circuits. The spectral subtraction process is performed, e.g., by execution of an appropriate algorithm by processor circuitry. While a spectral subtraction process is sometimes effective to reduce noise levels, a spectral subtraction process also introduces distortion upon the informational signal. In some instances, the distortion introduced upon the informational signal is so significant that the utility of such a process is significantly limited. A spectral subtraction process is inherently a frequency-domain process and therefore necessitates a potentially significant signal delay when converting a time domain signal received by circuitry utilizing such a process into the frequency domain. Also, because such a process typically utilizes fast Fourier transform techniques, the resolution permitted of practical circuitry which performs such a process is typically relatively low.
When the ratio of the level of the information signal is high relative to the level of the noise, such noise suppression process, in spite of these problems is typically fairly successful. However, when the ratio is high, there is also less of a need to perform noise suppression. Such a spectral subtraction process is therefore sometimes of a limited utility to significantly improve the quality of communications.
A radiotelephonic communication system is exemplary of a wireless communication system in which noise superimposed upon an informational signal affects the quality of communications transmitted during operation of the communication system. Noise can be superimposed upon the informational signal at any stage during the transmission and reception process including noise superimposed upon an informational signal prior to its application to the transmitter. Such noise can deleteriously affect the quality of communications.
A radiotelephonic device used in such a system is popularly utilized by a user when the user is also operating, or positioned in, a motor vehicle. Because no wireline connection is required between the radiotelephonic device and the infrastructure of a radiotelephonic communication system, communications can be effectuated between the radiotelephonic device and the infrastructure of a radiotelephonic communication system, communications can be effectuated between the radiotelephonic device and the infrastructure as the motor vehicle travels throughout any location encompassed by the infrastructure.
Conventional filter circuits, including those which perform a spectral subtraction process, can also be used to filter noise superimposed upon an informational signal applied to a radiotelephonic device. However, such conventional filter circuits sometimes introduce unacceptable levels of distortion upon the signal when it is filtered. Also, such conventional filter circuits are relatively slow and are of relatively low resolution as the spectral subtraction process is a frequency-domain process, typically utilizing fast Fourier transform techniques.
Noise having periodic characteristics is particularly problematical in radiotelephonic communication systems due to the popular utilization by users of radiotelephonic devices while operating motor vehicles. Engine sounds generated during operation of a motor vehicle can be superimposed upon an informational signal, i.e., a voice signal, formed when the user speaks into the microphone of the transmitter portion of the radiotelephonic device. Other noise generated by other noise sources, such as rotations of vehicular tires as the motor vehicle travels can also be superimposed upon the informational signal.
The engine sounds are periodic, having harmonic frequencies related to the frequencies at which the engine operates. The tire-rotation sounds are also periodic. Because the noise sometimes cannot be suppressed without introducing distortion upon the informational signal, decisions are sometimes made not to utilize a spectral subtraction process. By disabling or otherwise not utilizing circuitry which performs a spectral subtraction process, the periodic noise is not properly suppressed. Therefore, communication signals transmitted by the radiotelephonic device oftentimes are formed of, in addition to the informational signal, significant component portions caused by the superposition of the periodic noise signals upon the informational signals. Noise suppression circuitry forming a portion of receiver circuitry of the radiotelephonic system infrastructure also does not typically adequately remove or suppress such noise.
A manner by which to suppress periodic noise superimposed upon an informational signal would therefore be advantageous.
It is in light of this background information related to noise suppression circuitry and methods that the significant improvements of the present invention have evolved.